Applications that attempt to detect authorized and unauthorized video content of a received signal often rely on processes that analyze the content of the received signal to generate some type of content identifier or signature. These applications use results from the analysis to determine whether the received content is a copy of some reference content. For many of these applications, it is important to obtain a reliable identification even when the content of the received signal has been modified unintentionally or intentionally so that it differs from the reference content but can still be recognized by a human observer as being substantially the same as the reference content. If the perceived difference between the reference content and the modified content is small, then preferably the signature-generation process should generate signatures from the reference and modified content that are very similar to one another.
Examples of unintentional modifications to signal content include the insertion or addition of noise to signals in transmission channels and on storage media. Examples of intentional modifications to video signals include luminance and color modifications such as contrast/brightness adjustments, gamma correction, luminance histogram equalization, color saturation adjustments and color correction for white balancing, include geometric modifications such as image cropping and resizing, image rotation and flipping, stretching, speck removal, blurring, sharpening and edge enhancement, and include coding techniques such as lossy compression and frame rate conversion.